The different types of communication signals are typically classified according to certain basic characteristics, such as whether the signal carrier is amplitude modulated, angle modulated, pulse modulated, or some combination of these modulation types. In a pulse-modulated signal, the pulses may likewise be modulated in amplitude, duration (width), position (phase), repetition rate (interval), or any combination thereof. There may be a single channel or multiple channels of either analog, quantized or digital information modulating the carrier, and there may be various forms of multiplexing the information. The information may represent audio sound, video pictures, measurements, alphanumeric characters and symbols, other kinds of data, or some combination of these.
Pulse-duration modulation (PDM), also known as pulse-width modulation, is modulation of a pulse carrier wherein the value of the information signal modulating the carrier produces a pulse of proportional duration by varying the leading, trailing, or both edges of the pulse. Usually in PDM, the pulse spacing or interval remains constant. Pulse-spacing (or interval) modulation is a form of frequency modulation in which the spacing or interval between pulses is modulated in accord with the information value. Usually, the pulse duration or width remains constant in this modulation type. Pulse-position modulation (PPM) is phase modulation of a pulse carrier wherein the information value varies the position in time of a pulse relative to its unmodulated time of occurrence. This differs from pulse-spacing modulation in that PPM usually requires a reference clock in order to accurately judge the relative phase or position of the pulses.
“Keying” is any form of digital modulation in which the signals are formed by modulating any characteristic of the carrier between discrete values. On/off keying is a binary form of amplitude-shift keying with two discrete states, one of which is the presence and the other the absence of energy in the keying interval. The information might be represented by the duration of one of the states (e.g., dots and dashes in Morse code telegraphy). However, usually the amplitude state itself or the transition from one state to the other represents the encoded information. There are a variety of possible coding schemes (e.g., unipolar, polar, dipolar, return-to-zero, return-to-one, non-return-to-zero). For example, non-return-to-zero (NRZ) is a modulation mode in which it is not necessary for the signal to return to zero after each data element is encoded in the signal, whereas return-to-zero and return-to-one are modulation modes in which the signal does return to zero (or one) after each data element is encoded.
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulated output signal shifts between two or more discrete predetermined frequencies in accord with the information value. In multiple-frequency-shift keying (MFSK), groups of n data bits are encoded by 2n discrete frequencies. Phase-shift keying (PSK) is a form of phase modulation in which the modulating information shifts the instantaneous phase of the modulated signal between predetermined discrete phase values. Differential phase-shift keying (DPSK) is a form of PSK in which the reference phase for a given keying interval is the phase of the signal during the preceding keying interval. FSK and PSK modulation typically involves a continuous-wave carrier rather than pulses.
Multi-bit or N-ary (ternary, quaternary, octary, etc.) encoding schemes have modulated signals in which each signal condition (amplitude, frequency, phase) represents more than one bit of information. MFSK is one example, but PSK or DPSK could likewise have more than two discrete phases, e.g., 4 phases representing dibits.
Each of the various forms of modulation has its own set of advantages and disadvantages relative to a specific application for which it will be used. Some factors to consider in choosing a particular form of modulation include bandwidth, power consumption requirements, and the potential for signal propagation errors and recovery of the original information. For digital data, whether a separate clock signal is required or the modulated signal is self-clocking may be important. The relative simplicity or complexity of the modulating and demodulating equipment or circuitry may also be a factor in the decision. Low power consumption is particularly sought for use with capacitive-loaded transmission lines.